Wood is composed of two main parts, namely, a fibrous carbohydrate or cellulosic portion and a non-fibrous portion comprising a complex chemical, commonly referred to as lignin.
For use in papermaking processes wood must first be reduced to pulp, which can be defined as wood fibers capable of being slurried or suspended and then deposited as a screen to form a sheet. The methods employed to accomplish this pulping usually involve either physical or chemical treatment of the wood, or perhaps some combination of the two processes, to alter its chemical form and to give desired paper properties.
In chemical pulping, the wood chips are digested with chemical solutions to solubilize a portion of the lignin and the effect its removal of the lignin. The more usual of these digestive procedures are the sulfite, sulfate or Kraft, modified sulfite and soda processes.
Common industrial practices for the production of fully bleached chemical pulp grades entail two major unit operations namely 1) Cooking to transform wood chips into pulp and to dissolve associated lignin content, from more than 20% lignin in wood chips to less than 5% lignin in pulp and 2) Bleaching to substantially remove residual lignin in pulp and to increase the pulp brightness.
It is well known that from identical wood species and cooking to similar residual lignin content, pulps produced with alkali pulping processes such as Kraft are stronger but darker in color and more difficult to bleach than pulps produced under acidic conditions such as those used for acid sulfite pulping process.
Caustic, carbonate, sulfide, sulfite, chemical additives or any of their combination are known to be used for alkali pulping processes. Pulps with lower lignin content and higher brightness can also be produced through combination of alkali pulping and an extended delignification stage using bleaching chemicals such as hydrogen peroxide, oxygen or ozone.
Use of solutions of SO2, MHSO3 and M2SO3 on wood chips is well known in sulfite pulping industry (Pulp and Paper Manufacture Vol. 4. Sulfite Science & Technology, book published by Canadian Pulp and Paper Association). Sulfonation or use of solution of SO2 or combination of SO3 and SO2 on pulps has also been investigated as means to increase associated paper making properties (K. Kringstad, J. Olausson, Svensk Papperstidning n. 13, P 480-485, 1974) (U.S. Pat. No. 5,522,967). Use of SO2 as an alternative source of acid for pulp washing and trace metal removal has also been proposed in U.S. Pat. No. 3,725,194, and U.S. Pat. No. 4,560,437. However, none of these cited references disclose or teach SO2 as a treatment to remove lignin or to transform the treated lignin (i.e., modified lignin) in a manner that would be receptive to chemical reaction or substitution with bleaching chemical during the bleaching sequence.